Magnetospheric Electric Fields
at Mercury
Lars G. Blomberg
Space and Plasma Physics
School of Electrical Engineering
Royal Institute of Technology (KTH)
Stockholm
MESSENGER BepiColombo Workshop, Boulder, CO, USA, 4 November 2010
Outline
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Background
DC fields
Cross polar potential & Convection
Scaling
Field-aligned currents & Current closure
Charge flow
ULF waves / “field line resonances”
AC fields
“Kilometric” & Synchrotron Radiation
Outline
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Background
DC fields
Cross polar potential & Convection
Scaling
Field-aligned currents & Current closure
Charge flow
ULF waves / “field line resonances”
AC fields
“Kilometric” & Synchrotron Radiation
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Most topics are still unconstrained by measurements!
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Schematic of Mercury's
magnetosphere.
From Ip, 1986.
Solar wind at Mercury:
v = 300-800 km/s
ne = 30-70 cm-3
B = 20-40 nT
Ti = 11-16 eV
Te = 17-21 eV
Dipole moment:
200-400 nT RH3
Magnetopause stand-off:
1.0-1.5 RH
Estimate varies
Solar wind hits surface
depending on
an estimated 7% of the
assumptions regardtime.
ing higher order terms.
Electric potential
across magnetosphere:
5-25 kV
Cross-tail E field:
1-5 mV/m
DC & low frequency
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Plasma convection electric field
Electric fields related to field-aligned currents and
their closure
Pulsations driven by solar wind interaction with
the magnetosphere
Inductive electric fields during tail reconfiguration
Higher frequencies
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“Kilometric” radiation from acceleration
regions
Synchrotron radiation
DC electric field
Solar Wind
v ~ 420 km/s
B ~ 21-46 nT
Typical electric field: 9-20 mV/m
DC electric field
Solar Wind
v ~ 1200 km/s
B ~ 40 nT
ACE data
from 31
October
2003
DC electric field
Solar Wind
v ~ 1200 km/s
B ~ 40 nT
ACE data
from 31
October
2003
B scaled to Mercury: 140 nT at aphelion, 300 nT at perihelion
DC electric field
Solar Wind
v ~ 1200 km/s
B ~ 300 nT
Extreme electric field: 360 mV/m
DC electric field
Solar Wind
v ~ 1200 km/s
B ~ 300 nT
Extreme electric field: 360 mV/m
Also noteworthy:
300 nT solar wind magnetic field is comparable
to the surface flux density of Mercury’s intrinsic
magnetic field !!
DC electric field
Magnetosphere
typical Esw ~ 9-20 mV/m
cross-section ~ 7000 km
Solar wind potential drop ~ 60-140 kV
DC electric field
Magnetosphere
typical Esw ~ 9-20 mV/m
cross-section ~ 7000 km
Solar wind potential drop ~ 60-140 kV
10-20% penetration => cross polar potential ~ 5-25 kV
DC electric field
Magnetosphere
typical Esw ~ 9-20 mV/m
cross-section ~ 7000 km
Solar wind potential drop ~ 60-140 kV
10-20% penetration => cross polar potential ~ 5-25 kV
But: can we assume that the reconnection efficiency /
penetration fraction at Mercury is similar to Earth’s?
Cross polar potential saturation
Φ pc
ΦrΦs
=
Φr + Φs
where
Φpc is the cross polar potential,
Φr is the reconnection voltage, and
Φs is the saturation potential
Cross polar potential saturation
Φ pc
ΦrΦs
=
Φr + Φs
where
Φpc is the cross polar potential,
Φr is the reconnection voltage, and
Φs is the saturation potential
However, this likely requires a
dayside FAC system
The terrestrial cross polar
potential has been observed
to saturate for both
southward (two-cell
convection) and northward
(four-cell convection) IMF.
What about Mercury?
Does the cross polar potential
saturate or not?
Does four-cell convection /
lobe reconnection happen at
all? If so, what do the
associated current systems
look like?
Sundberg et al, J. Geophys. Res., 2009
Does even stable two-cell
convection exist?
Two-fluid simulation of plasma convection during
1st MESSENGER flyby
Benna et al., Icarus, 2010
Scaling considerations
Even though the convection electric field is likely larger
at Mercury than at Earth, the integrated potential across
the magnetosphere is significantly smaller
Similarly, the inductive electric field associated with tail
reconfiguration is likely of the same order at Mercury as
at Earth, but the integrated “loop voltage” is much
lower
Thus, acceleration processes dependent on potential
drops or voltages will be less effective at Mercury
Field-aligned currents
Are they persistent or
transient?
If transient, are they part of
closed current circuits or
due to redistribution of net
charge?
Do they play a role in
substorms?
Magnetometer signatures of field-aligned currents at
Mercury. After Slavin et al., 1997.
What is the power
dissipation at the load?
How do they close at or
near the planetary surface?
Current closure
No conducting ionosphere
Poorly conducting photoelectron cloud on the dayside
Planetary surface may
conduct some current, but
coupling to the environment
difficult on the nightside
Current closure near or through planetary surface.
After Grard et al., 1997
Low mobility of charged
particles on the nightside
surface may give charge pileup and localized discharges
Current Closure
Magnetic field from field-aligned current:
jz = (∇ ⋅ H ) z =
∂H y
∂H x
−
∂x
∂y
Current continuity in closure region:
jz = ∇ ⋅ (J P + J H )
Current Closure (2)
∂H y
∂H x
(∇ ⋅ H ) z =
−
= ∇ ⋅ (J P + J H )
∂x
∂y
J = ΣE
and assuming ∇ ⋅ J H = 0
∂
∂
⇒ ( H y − Σ P Ex ) − ( H x + Σ P E y ) = 0
∂x
∂y
Current Closure (3)
Hy
Hx
Σ P = =−
Ex
Ey
Current Closure (3)
Hy
Hx
Σ P = =−
Ex
Ey
assuming instead ∇ ⋅ J P = 0
Hy
Hx
⇒ ΣH = −
=−
Ex
Ey
Current Closure (4)
When FAC close via Pedersen currents
orthogonal components of the horizontal E & B are
correlated
When FAC close via Hall currents aligned
components of the horizontal E & B are correlated
For closure in a photo-electron cloud, the ratio of
“life time” to gyro period will determine whether
the closure current is a Pedersen or a Hall current.
Interhemispheric charge flow
There are indications of the magnetic
dipole being offset somewhat from the
centre of Mercury (or alternatively, the
quadrupole moment is significant).
This would mean that all particles
escaping the surface at B1 will reach
the surface at B2, whereas those
particles leaving the surface at B2 with
a high pitch angle will mirror before
reaching B1 and return to B2.
B1
B1 > B2
B2
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Unless the charge flow is associated with a
stationary current, a weak field-aligned electric
field will arise
Also, there have been speculations about
negative charge pile-up on the nightside possibly
giving rise to localized strong potential wells
(kilovolts?)
Because of photo-emission, the dayside surface
will charge positively, but to a much lesser extent
(volts)
ULF Waves /
“Field line resonances”
Driven by Kelvin-Helmholtz
instabilities at the flanks of
the magnetopause?
Period depends on magnetic
field and plasma density.
E-B phase relationship
depends on conductivity at
reflective interface (at or
near planetary surface).
“Field-line resonances” detected by Mariner 10.
After Russell, 1989.
May need multi-ion
treatment. Possibility to
determine sodium content?
(Othmer et al., 1999)
Likely compressional
(Kim et al., 2008)
MESSENGER 3rd flyby magnetometer data
Kelvin-Helmholtz waves studied by:
Boardsen et al. (Geophys. Res. Lett., 2010)
Sundberg et al. (Planet. Space Sci., submitted manuscript)
“Field line resonances”
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In contrast to Earth, they are not regular MHD
waves at Mercury since the transverse scale
length is typically less than an ion gyro radius
Compressional rather than transverse?
Bounce periods of the order few - tens s to be
expected
May help understand reflective/conductive
properties of surface (or other possible reflection
regions below the spacecraft), since reflection
coefficient depends on conductivity mismatching
AC electric field
Solar Wind
fce ~ 0.5-10 kHz
fpe ~ 50-100 kHz
AC electric field
Solar Wind
fce ~ 0.5-10 kHz
fpe ~ 50-100 kHz
Harmonic Langmuir waves may exist in the
foreshock region, up to several times fpe
AC electric field
Magnetosphere
fce < 11 kHz
(B < 400 nT)
fcH+ < 6 Hz, fraction of Hz for heavier ions
AC electric field
Magnetosphere
fce < 11 kHz
Mukai et al. (2004) estimates plasma sheet
densities in the range 0.1-32 cm-3
fpe ~ 3-50 kHz
“HKR”
“Auroral”
acceleration???
Auroral like parallel electron
acceleration is associated
with kilometric radiation.
“Auroral”
acceleration
region
electrons
Emitted at local cyclotron
frequency, in the case of
Mercury of the order of a few
kHz.
Excellent remote sensing tool
for acceleration regions, also
in the absense of auroral
emissions.
Possible problem with
mixing due to wave reflection
at magnetopause.
Synchrotron Radiation
Upper limit set by size of magnetosphere
ρ
=
γ mv
eB
≈
γ mc
eB
ρ= 500 km, B= 100 nT ⇒ γ = 29
corresponding to 14 MeV electron energy
f peak
eB  Wk 
=


2π m  W0 
2
Synchrotron Radiation (2)
fpeak = 2.3 MHz
 500 km is likely an upper, upper limit to the
possible gyro radius
 300 km may be more realistic, corresponding
to fpeak ≈ 1 MHz
 or even less, if it at all occurs...
BepiColombo Mercury Magnetospheric Orbiter (MMO)
Plasma Wave Investigation (PWI)
Sensors & Receivers
Electric Field Sensors (32m tip-to-tip dipoles):
WPT (Wire-Probe anTenna)
MEFISTO (Mercury Electric Field In-Situ TOol )
Magnetic Field Sensors (search-coils)
LF-SC (Low-Frequency Search-Coils, 2 sensors)
DB-SC (Dual-Band Search-Coils, 1+1 sensors)
DC/Low frequency Electric field (0Hz ～32Hz)
EWO-EFD (Electric Field Detector)
Low/medium frequency E/B field (10Hz ～ 120kHz (E), 0Hz ～20kHz(B))
EWO-WFC/OFA (WaveForm Capture/Onboard Frequency Analyzer)
High frequency E/B field (2.5kHz～10MHz (E), 2.5kHz～640kHz (B))
SORBET (Spectroscopie Ondes Radio & Bruit Electrostatique Thermique)
Active Impedance Measurement
AM2P (Active Measurement of Mercury’s Plasma )
Animation
courtesy
RISH,
Kyoto
University
Possible waves in the Mercury environment
From Matsumoto et al., 2006
Concluding Remarks
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Electric fields never directly measured at Mercury
DC – MHz expected in Mercury’s magnetosphere
Some terrestrial phenomena will be very similar at
Mercury while others may not occur at all or be
very different
Size of magnetosphere often deciding factor
First measurements planned for 2021...
Concluding Remarks

Electric fields never directly measured at Mercury
DC – MHz expected in Mercury’s magnetosphere
Some terrestrial phenomena will be very similar at
Mercury while others may not occur at all or be
very different
Size of magnetosphere often deciding factor
First measurements planned for 2021...
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Most predictable thing: we will be surprised!
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